Methods for drying and cleaning of objects using aerosols and inert gases

ABSTRACT

Methods for cleaning and/or drying objects that may have been wetted or contaminated in a manufacturing process. The objects are submerged in a rinse liquid in an enclosed chamber, and aerosol particles from a selected liquid are introduced into the chamber above the rinse liquid surface, forming a thin film on this surface. As the rinse liquid is slowly drained, some aerosol particles settle onto the exposed surfaces of the objects, and displace and remove rinse liquid residues from the exposed surfaces, possibly by a &#34;chemical squeegeeing&#34; effect. Surface contaminants are also removed by this process which may be performed at about room temperature. Chamber pressure is maintained at or near the external environment pressure as the rinse liquid is drained from the chamber. Inert gas flow is employed to provide aerosol particles of smaller size and/or with greater dispersion within the chamber. Continuous filtering and shunt filtering are employed to remove most contaminants from the selected liquid. A flow deflector redirects initial flow of the selected liquid to a supplementary filter, to remove most of the contaminant particle &#34;spike&#34; that appears when a system is first (re)activated. An improved surface for aerosol particle production is provided.

This patent application is a Continuation In Part of U.S. Ser. No.08/984,413, filed Dec. 3, 1997, which is a Continuation of U.S. Ser. No.08/624,689, filed Mar. 25, 1996, now abandoned, which is a ContinuationIn Part of U.S. Ser. No. 08/616,165, filed Mar. 14, 1996, now U.S. Pat.No. 5,685,086, which is a Continuation In Part of U.S. Ser. No.08/484,921, filed Jun. 7, 1995, now U.S. Pat. No. 5,653,045. Thisinvention relates to improvements in drying and cleaning of manufacturedobjects, including electronic components, using aerosols created bysonic or ultrasonic means.

FIELD OF THE INVENTION BACKGROUND OF THE INVENTION

Objects that are being manufactured using processes involvingapplication of liquids and other fluids often require that the parts bethoroughly dried before the manufacturing process can continue. Forexample, in fabrication of integrated circuits, doping, photomasking,etching and passivation processes often require application ofparticular liquids at one stage and removal of liquid residues beforethe next stage proceeds. Drying and removal of these liquid residuesmust be complete, but the drying process should, ideally, occur in arelatively short time interval and with expenditure of a minimum ofenergy and chemicals to implement the drying process.

Several workers have disclosed methods for drying parts, includingintegrated circuits, by use of heated or superheated gases. Theseapproaches heated or superheated gases or direct beam irradiation to dryan object surface; or they use cooperative action by an ultrasonic beamand an active chemical bath to remove contaminants from, or to apply adesired material to, an object surface. These approaches are complex,usually require operation at high temperatures, often require processingtimes of several minutes, and often require use of specially resistantchamber walls for the processing chamber.

What is needed is a method and associated apparatus for drying andcleaning objects in a manufacturing process that works well at roomtemperature and is simple, that is demonstrably complete, with nosignificant residues, that can be accomplished in times as short as oneminute, that can be performed in a chamber with chamber walls made ofalmost any material, and that requires use of only a very small amountof a drying agent, with minimal expenditure of energy, particularlythermal energy. Preferably, the process should be performable over awide range of temperatures, and should be easily scalable to any sizesurface.

SUMMARY OF THE INVENTION

The needs are met by the invention, which provides improvements for amethod and associated apparatus for drying and/or cleaning objects byuse of a small amount of a low surface tension liquid plus (optionally)brief application of a recyclable cleaning agent. In one embodiment, theobjects to be dried are submerged in a rinse liquid, such as water, in achamber. The rinse liquid surface is covered with a very thin film of alow surface tension selected liquid, such as isopropyl alcohol ("IPA"),formed from an aerosol created by sonic or ultrasonic vibrations of asmall stream of the selected liquid. Other suitable liquids includeethyl alcohol, methyl alcohol, tetrahydrofuran, acetone,perfluorohexane, hexane and ether. The thin film is continuallyreplenished as needed, and the rinse liquid covering the objects to bedried is slowly drained. As the rinse liquid and thin film drain, theselected liquid briefly contacts the surfaces of the objects and removeswater residues by a "chemical squeegeeing" process that is discussedlater. Optionally, the objects can be subjected to an additional chamberpurge or drying process, using a heated or ambient temperature cleaningfluid, such as dry N₂ CO or CO₂ gas, after the chamber has been drained.Optionally, chamber pressure is maintained near or above the externalenvironment pressure as the rinse liquid is drained from the chamber.

In a first improvement, high velocity flow of a supplemental gasprovides a controllable expansion of the selected liquid and produces a"fog" with reduced aerosol diameters and improved drying and/or cleaningaction.

In a second improvement, controlled flow of a supplemental gas, combinedwith a mask that captures the heavier selected liquid particles (e.g.,those with aerosol diameters>10 μm), produces a fog with reduced aerosoldiameters and improved drying and/or cleaning action.

In a third improvement, the selected liquid is continuously circulatedand filtered to continually remove substantially all contaminants withdiameters greater than a selected value, such as 0.05 μm.

In a fourth improvement, a shutter is used on the delivery system forthe selected liquid to suppress or substantially eliminate contaminantparticle "spikes" that occur during a start-up phase of the deliveryapparatus.

In a fifth improvement, an improved aerosol particle production system,using an inert particle formation surface, is provided that providesimproved control of aerosol diameters and allows production of muchsmaller diameters.

Process parameters that can be varied to control the process includevibration frequency for creation of aerosol particles from the selectedliquid, a representative aerosol particle diameter, delivery rate forthe selected liquid, pressure and temperature at which the selectedliquid is delivered for creation of the aerosol particles, temperatureof the drying fluid used (if any), and choice of the selected liquid andof the drying fluid used (if any).

The invention requires as little as 1-2 milliliters (ml) of the selectedliquid to dry objects in a chamber with volume of 10-20 liters, orsmaller or larger, if desired. This approach provides several benefits.First, the process is carried out at or near room temperature, withlittle energy expenditure, and does not require use of heated orsuperheated liquids or gases for drying. Second, the process uses a verysmall amount of the selected liquid in a large volume of rinse liquid(10-20 liters) so that the mixture of rinse liquid and selected liquidcan normally be disposed of without the special handling proceduresrequired for hazardous materials. Third, a wide variety of inexpensiveselected liquids can be used. Fourth, use of a covering film of selectedliquid minimizes vapor from the rinse liquid remaining in the chamberafter drainage. Fifth, the process is easily scaled up or down, with nosubstantial changes in the apparatus. Sixth, the process removes largediameter contaminants that are not chemically bound to an objectsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates suitable apparatus, in one embodiment, for practicingthe invention, with the objects submerged in a rinse liquid in achamber.

FIGS. 2A, 2B, 2C and 2D are schematic views of aerosol creatingvibrating nozzles suitable for use with the invention.

FIG. 2E is a graphical view of inert gas plenum pressure variations withtime that are suitable for use with the apparatus in FIG. 2D.

FIG. 3 illustrates the apparatus of FIG. 1 with the rinse liquid partlydrained from the chamber.

FIG. 4 is a flow chart of one embodiment of the method.

FIG. 5 is a schematic view of continuous filtering apparatus for aselected liquid according to the invention.

FIG. 6 is a schematic view of shutter apparatus that may be used with anaerosol production delivery system according to the invention.

FIG. 7 is a schematic view of aerosol production apparatus according tothe invention.

DESCRIPTION OF BEST MODES OF THE INVENTION

FIG. 1 illustrates one embodiment of apparatus 10 that is useful forpracticing the invention. An enclosed chamber 11 is defined by a housing12 and is provided with a rack (optional) for holding the objects 13A,13B, 13C, etc. to be dried. The objects 13A, 13B, 13C are placed into,and removed from, the chamber 11 through a slidable, hinged or otheroperable entryway 15 that is part of the housing 12. When the entryway15 is closed or engaged, the chamber is enclosed, preferably in angas-tight manner, and any remaining gas in the chamber can optionally beremoved. A first port 21 and associated first valve 23 are attached tothe housing 12 and are connected to a source 25 of water or othersuitable rinse liquid 27 in which the objects 13A, 13B, 13C areinitially submerged. A second port 31 and associated second valve 33 areattached to the housing 12 and are connected to a selected liquid source35, such as a pressurized tank maintained at a pressure of 5-50 psi, ofa selected drying liquid or fluid 37 ("selected liquid") that willprimarily dry the objects 13A, 13B, 13C.

A third port 41 and associated third valve 43, which may coincide withthe first port 21 and first valve 23, are attached to the housing 12 andare connected to a first liquid or fluid tank or other suitable firstdrain acceptor 45 that receives and drains the rinse liquid 27 andabsorbed selected liquid 37 from the chamber 11. A fourth port 51 andassociated fourth valve 53, which may coincide with the second port 31and second valve 33, are attached to the housing 12 and are connected toa second liquid or fluid tank or other suitable second drain means 55that receives and drains the selected liquid 37, and aerosol droplets 39from the selected liquid, from the chamber 11.

Initially, the objects 13A, 13B, 13C are placed in the chamber 11 in arack or cassette (not shown), the entryway 15 is closed or engaged, thechamber has a pressure at or slightly above atmospheric, and rinseliquid 27 is admitted to the chamber through the first port 21 and firstvalve 23 so that the objects are fully submerged in the rinse liquid.The first valve 23 is then closed. Alternatively, the objects 13A, 13B,13C may be partly submerged in the rinse liquid 27 so that a portion ofthe surfaces of these objects are exposed above the exposed surface ofthe rinse liquid.

A small stream of the selected liquid 37 then passes through the secondport 31 and second valve 33 and is received by a piezoelectricallydriven head 61 and vibrating sonic or ultrasonic nozzle 63 that vibratesat a selected frequency f lying in the range 10 kHz≦f≦10,000 kHz, andmore preferably in the narrower range 20 kHz≦f≦100 kHz. The driven head61 is connected to and driven by a frequency generator 64 that ispreferably located outside the chamber 11 and that permits selection ofa vibration frequency f in the indicated range. When the selected liquid37 is present in the vibrating nozzle 63 and the nozzle is vibrating,the selected liquid is converted into a plurality of aerosol droplets 39that move into the chamber 11 and occupy most or all of an upper portion11U of the chamber that is not already filled by the rinse liquid 27 andsubmerged objects 13A, 13B, 13C.

FIG. 2A illustrates a suitable drive head 61A and vibrating nozzle 63Athat can be used with the apparatus shown in FIG. 1. The vibratingnozzle 63A preferably has a hollow column 65A formed therein withdiameter d(col)≈200 μm, through which the selected liquid 37(cross-hatched) flows. The vibrating nozzle then "shakes off" smalldroplets 39 of selected liquid 37, which form aerosol droplets in aroughly cylindrical pattern and move into the portion of the chamber 11above the rinse liquid.

FIG. 2B illustrates another suitable drive head 61B and vibrating nozzle63B, including a thin hollow column 65B therein through which theselected liquid 37 flows. A housing 67B surrounds the nozzle 63B anddirects a ring of hot or cold inert gas 69B toward the aerosol droplets39, which move into the chamber in a conical or other desired patternfor enhanced distribution of the aerosol droplets throughout thechamber. Many other drive head/vibrating nozzle combinations can also beused here.

I have found that use of a higher frequency f will tend to produceaerosol droplets 39 with a smaller mean diameter d(mean). For avibration frequency f in the range 20 kHz≦f≦100 kHz, I estimate that themean aerosol droplet diameter lies in the range 10 μm≦d(mean) ≦50 μm.The mean droplet diameter can be varied by varying the diameter(s)d(mem) of the membrane apertures 66 and by varying the frequency f ofvibration of the vibrating nozzle 63A or 63B.

The selected liquid 37 should be non-reactive with the objects 13A, 13B,13C and with the walls of the chamber 11 and should have a substantiallylower surface tension than the surface tension of the rinse liquid.Suitable selected liquids include isopropyl alcohol, ethyl alcohol,methyl alcohol, tetrahydrofuran, acetone, perfluorohexane, hexane andether, as well as many other low surface tension liquids and fluids. Useof any of these substances as a selected liquid does not requireprovision of chamber walls made of specially-resistant materials.

The selected liquid 37 may be held in the selected liquid source 35 at apressure of 5-50 psi above atmospheric pressure to facilitate deliveryand to suppress the slight volatilization of the selected liquid thatmight otherwise naturally occur. The preferred rinse liquid, de-ionizedwater, has a surface tension σ=73 dynes/cm at T≈20° C., and organicmolecules such as methyl alcohol, ethyl alcohol, isopropyl alcohol,n-hexane and ether have surface tensions a in the range 17 dynes/cm≦σ≦23dynes/cm at T=20° C. so that σ(selected liquid) <<σ(rinse liquid) atroom temperature.

Use of the selected liquid 37 at or near room temperature is preferredhere. Use of the selected liquid 37 at a substantially elevatedtemperature can reduce the surface tension of the rinse liquid 27,relative to the surface tension of the selected liquid 37, and thusinterfere with the chemical squeegee effect relied upon for thisprocess.

An aerosol particle is a cluster or collection of molecules of theselected liquid 37 that has not undergone a phase transformation into avapor form. Thus, the energy E(aerosol) (1.6 Watts for a typical sonichead, or less than 100 Joules/gm at a flow rate of 2 ml/min) required toconvert one gram of the selected liquid 37 into aerosol droplets 39,provided by the vibrating nozzle, is much less than the energy ofvaporization E(vapor) required to heat and convert one gram of theselected liquid 37 into its vapor form. We estimate that the ratioE(aerosol)/E(vapor) is less than 2 percent. Production of the aerosolparticles can be carried out at or near room temperature, and use of avery high temperature, such as T=60-200° C., is neither needed noradvisable for this process. Further, only a small amount of the selectedliquid 37, as low as 1-5 ml, is required for drying several objects 13A,13B, 13C in the chamber 11.

The aerosol droplets 39 move into the chamber 11, and many of thesedroplets settle onto an exposed surface 29 (preferably calm) of therinse liquid 27 as a thin film 30 having a varying thickness h(aerosol).An estimated time required to form this thin film 30 is 40-60 sec. Aportion of the aerosol droplets 39 that join the film 30 will diffuseinto the rinse liquid 27 so that, if this film is not replenished withadditional aerosol droplets, the film 30 will quickly and substantiallydisappear. Preferably, the volume flow rate r(sel) of the selectedliquid 37 to the vibrating nozzle 63 is adjusted so that the rate atwhich aerosol droplets 39 join the film 30 is sufficient to maintain orincrease a selected thickness h(aerosol) for the film. A preferred rangefor the film thickness h(aerosol) is 0.5 mm≦h(aerosol)≦5 mm, but thisthickness may be made larger by increasing the volume flow rate r(sel).For a chamber 11 having an exposed (upper) surface for the rinse liquid27 with an area of about 900 cm², a volume flow rate r(sel)=r2=1-5 mlper minute of the selected liquid 37 suffices. Usually, a volume flowrate r2=1-2 ml/min is high enough. The time required to drain thechamber at a drain rate of 5 mm/sec is about 20-40 sec for asemiconductor wafer 10-20 cm in diameter. Thus, very little of theselected liquid 37 is absorbed or diffuses into the rinse liquid 27 inthe course of the time interval (60-100 sec) required for establishmentof the film and draining of the chamber.

Because so little of the selected liquid 37 is used in the process, theselected liquid source 35 may have a relatively small volume, as littleas 20-25 ml, and the selected liquid source 35 may be located at aconsiderable distance, such as 1-4 meters, from the chamber 11. Thisenhances the safety of the process, where a selected liquid is used thathas a low flash point or that can initiate an explosion.

A very small amount of the selected liquid 37 will vaporize naturally atthe process temperature, preferably room temperature, based on theequilibrium vapor pressure coefficient of the selected liquid at thattemperature. This vaporized portion should be relatively small in theenclosed chamber 11 at room temperature, and the vapor portion of theselected liquid 37 will quickly come to equilibrium with the liquid filmand aerosol portions of the selected liquid 37. Use of a processtemperature much higher than room temperature would produce a selectedliquid 37 with a moderately higher equilibrium vapor pressurecoefficient and a comcomitantly higher amount of vapor from the selectedliquid. This natural vaporization of a small part of the selected liquid37 is not regarded as a useful part of the drying process.

After a film 30 of the aerosol droplets is established on the surface 29of the rinse liquid 27, which may require 40-60 sec, the rinse liquid 27is slowly drained from the chamber 11 through the third port 41 andthird valve 43 into the drain tank 45. Draining of the rinse liquid 27will require an estimated 20-40 sec for a chamber holding 10-20 litersof the rinse liquid 27. A preferred range for the drain rate r(drain) is3-10 mm/sec decrease in the height of the rinse liquid 27 in the chamber11, and r(drain)=5 mm/sec is a suitable drain rate for this process.Draining occurs slowly in order to preserve the thin film 30 of theselected liquid 37 at the otherwise-exposed surface 29 of the rinseliquid. As draining of the rinse liquid 27 proceeds, aerosol droplets 39continue to be produced by flow of a small stream of the selected liquid37 through the vibrating nozzle 63. The volume flow rate r(sel) of theselected liquid 37 may be adjusted toward higher or lowers values asdraining of the rinse liquid 27 (and absorbed aerosol particles 39)proceeds.

FIG. 2C illustrates improved ultrasonic drive apparatus for delivery ofa selected liquid, 117 and 119 (e.g., IPA at room temperature) inaerosol form, for cleaning and/or drying an object. A firstfluid-carrying line 101 delivers a selected liquid from a selectedliquid source 103 to a first end of an axial chamber 105 that is definedby an approximately cylindrical ultrasonic nozzle 107 that vibrates at afrequency flying in a selected frequency range, such as 10 kHz≦f≦10,000kHz. The nozzle 107 is driven by an ultrasonic drive head 109, which mayinclude several current-carrying coils. As the direction of coil currentis reversed repeatedly, the nozzle 107, which may include apiezoelectric material, is set into vibration by the coil currentreversal and "shakes off" small aerosol particles of the selectedliquid.

The apparatus in FIG. 2C also includes a second fluid-carrying line 111,fed by an inert gas source 113, that delivers gas to an inert gas plenum114, with a plenum wall 115 having a small aperture 116 near a secondend of the axial chamber 105. The fluid-carrying line 111 preferablydelivers inert gas, such as N₂ or CO, to the plenum 114 at about roomtemperature at a flow rate of 2-10 liters per minute (LPM). A portion orall of the inert gas is expelled from the plenum 114 at the plenumaperture 116 in a selected (outward) direction, creating a locallylowered total gas pressure that results in an expanded flow 117 of theselected liquid aerosol particles. In the absence of the inert gas flowthrough the plenum aperture 116, only the selected liquid aerosolparticles 119 would appear, and only in a central region below a secondend of the nozzle 107 and surrounding a nozzle axis AA. With inert gasflow through the plenum aperture 116 incorporated, the aerosol particlesflow within a larger "cone angle" θ_(c),C than would occur in theabsence of the inert gas.

FIG. 2D also illustrates improved ultrasonic drive apparatus fordelivery of a selected liquid, 137 and 139, in aerosol form, forcleaning and/or drying an object. A first fluid-carrying line 121, aselected liquid source 123, an axial chamber 125 defined by anapproximately cylindrical ultrasonic nozzle 127, an ultrasonic drivehead 129, a second fluid-carrying line 131, an inert gas source 133, aninert gas plenum 134, a plenum wall 135, and a plenum aperture 136 servethe same purposes as the respective components 101, 103, 105, 107, 109,111, 113, 114, 115, and 116 in the apparatus of FIG. 2C. The embodimentshown in FIG. 2D will produce aerosol particles 137 with a largerassociated cone angle θ_(c),D than would occur in the absence of theinert gas.

In FIG. 2D, the inert gas is expelled from the plenum aperture 136,either continuously at roughly constant plenum pressure, or at a plenumpressure p(plenum) that varies roughly periodically with time t betweena minimum pressure value and a maximum pressure value, as indicated inFIG. 2E. This pressure variation creates inert gas pressure waves 141with an associated pressure gradient around the plenum aperture 136.These pressure waves 141 cause aerosol particles 117 spaced apart from acentral region of the nozzle 127 to separate into smaller clusters andthereby produce smaller aerosol particles and to expand outward within awider associated cone angle θ_(c),D. Aerosol particles 139 that areproduced with a central region near an axis of the nozzle 127 willretain their larger particle diameters and will be captured by one ormore particle masks or absorbers 141 and 143 that are located in acentral region below the nozzle 127. The selected liquid aerosolparticles 139 captured by the particle masks 141 and 143 are drainedinto a reservoir 144 and may be recycled if desired. The embodimentshown in FIG. 2D will produce aerosol particles 137 with a largerassociated cone angle θ_(c),D and with smaller average aerosoldiameters,because the larger diameter aerosol particles tend to moveparallel to a nozzle axis AA and tend to be captured by one or more ofthe centrally located particle masks 141 and 143.

As the rinse liquid 27 drains from the chamber 11 in FIG. 3, thesurfaces 14A, 14B, 14C of the objects 13A, 13B, 13C are increasinglyexposed above the exposed rinse liquid surface 29 and overlying film 30,and aerosol droplets 39 in the upper part of the chamber 11U settle ontothese exposed surfaces 14A, 14B, 14C, as shown in FIG. 3. Also, aportion of the film 30 of the selected liquid 37 may settle on theexposed portions of the object surfaces 14A, 14B, 14C, rather thanmoving with the rinse liquid 27 toward the third port 41. The selectedliquid 37 is chosen to have a much smaller surface tension σ(sel) thanthe surface tension σ(rinse) of the rinse liquid 27. If the rinse liquid27 is water, the associated surface tension is σ(rinse)=73 dynes/cm atroom temperature. In this instance, the selected liquid 37 may beisopropyl alcohol ("IPA") or ethyl alcohol or methyl alcohol, with therespective surface tensions of σ=21.7 dynes/cm, 22.6 dynes/cm, and 22.8dynes/cm at room temperature. The selected liquid 37 is also chosen forits ability to displace or solubilize rinse liquid at whatever processtemperature is used. Room temperature (T=20° C.), and even lowertemperatures, can be used here. The process also works satisfactorily atsomewhat higher temperatures.

As exposed portions of the object surfaces 14A, 14B, 14C receive theaerosol droplets 39 of the selected liquid 37, new films 16A, 16B, 16Cof the aerosol droplets 39 or selected liquid 37 form on these exposedportions. As draining of the rinse liquid 27 from the chamber 11proceeds, and after draining is completed, the selected liquid 37 in thefilms 16A, 16B, 16C displaces most or all of the rinse liquid 27 thatremains on the exposed portions of the object surfaces 14A, 14B, 14C, inlarge part because the surface tension σ(sel) of the selected liquid 37is much smaller than the surface tension σ(rinse) of the rinse liquid27. The rinse liquid 27 that is displaced by the selected liquid runsdown the exposed surfaces 14A, 14B, 14C of the objects 13A, 13B, 13C andis drained away with the bulk of the rinse liquid in the chamber. Theselected liquid 37 that forms a film on the surfaces 14A, 14B, 14C ofthe objects 13A, 13B, 13C also runs down these surfaces and is drainedaway with the bulk of the rinse liquid 27. The films 16A, 16B, 16C ofselected liquid 37 thus act as "chemical squeegees" in removing rinseliquid 27 and selected liquid 37 from the exposed surfaces 14A, 14B, 14Cof the objects 13A, 13B, 13C.

This chemical squeegeeing of the objects' exposed surfaces 14A, 14B, 14Chas another benefit. The process not only dries the objects' surfacesbut also removes most of the larger contaminant particles from thesesurfaces, if these contaminant particles are not chemically bound to thehost surfaces. I have examined some bare silicon surfaces before thechemical squeegeeing process is applied and have found a substantialnumber of contaminant particles with diameter at least 0.3 μm on thesesurfaces, as indicated in column (2) of Table 1. I have then applied thechemical squeegeeing process, have re-examined the same surfaces aftercompletion of the chemical squeegeeing process, and have found thenumber of contaminant particles is reduced after completion of thechemical squeegeeing process, as shown in column (3) of Table 1. Theseresults indicate that chemical squeegeeing alone removes 12-100 percentof the contaminant particles with diameters greater than 0.3 μm,depending on size.

                  TABLE 1                                                         ______________________________________                                        Chemical Squeegee Removal of Large Contaminant Particles                                       Particles before                                                                          Particles after                                    Particle Size Chem. Squeegee Chem Squeegee                                  ______________________________________                                        0.329-0.517 μm                                                                          8           7                                                      0.518-0.810 7 2                                                               0.811-1.270 7 2                                                               1.271-1.990 3 1                                                               1.991-3.130 6 1                                                               3.131-4.910 6 0                                                             ______________________________________                                    

At about the time the rinse liquid 27 becomes fully drained from thechamber 11 and the surfaces 14A, 14B, 14C of the objects 13A, 13B, 13Care fully exposed, the second port 31 and second valve 33 are closed,the vibrating nozzle 63 is shut down, and the fourth port 51 and fourthvalve 53 are opened. The remaining selected liquid 37, aerosol droplets39, rinse liquid 27, and any vapor from the rinse liquid and selectedliquid are then removed from the chamber 11 through the fourth port 51.This portion of the process may require another 10-20 sec. but may becontinued for a longer time interval, if desired, to completely removethe remaining selected liquid 37 and any remaining rinse liquid 27 fromthe films 16A, 16B, 16C and from the chamber 11. Drying of the objects13A, 13B, 13C is now substantially complete.

Optionally, hot or room temperature dry nitrogen N2, carbon monoxide CO,carbon dioxide CO₂ or other inert gas may be admitted into the chamber11 through a fifth port 71 and associated fifth valve 73 to purge thechamber 11 and/or clean any remaining substances from the exposedsurfaces 14A, 14B, 14C of the objects 13A, 13B, 13C. The hot purge gasis received by the chamber 11 from a purge gas tank 75 and is removedthrough a sixth port 81 and associated sixth valve 83 that may coincidewith the fifth port 71 and fifth valve 73, respectively. The hot purgegas is received from the chamber 11 in a spent purge gas tank 85 forrecycling, processing or disposal. This portion of the process, ifincluded, may require another 30-60 sec.

FIG. 4 is a flow chart indicating the process steps to be taken in oneembodiment of the invention. In step 91, the objects 13A, 13B, 13C(FIGS. 1 and 3) to be dried and/or cleaned are placed into the chamber,and the chamber is closed. In step 93, rinse liquid 27 is admitted intothe chamber to partially or (preferably) fully submerge the objects. Instep 95, aerosol droplets of the selected liquid 37 are formed withinthe chamber, and a film of the selected liquid is formed and maintainedon the exposed surface of the rinse liquid. In step 97, the rinse liquid27 and any absorbed selected liquid 37 are slowly drained from thechamber, to ultimately expose the surfaces of the objects to the aerosoldroplets and to allow films of the selected liquid to form on theobjects surfaces; optionally, the chamber pressure is maintained near orabove the external environment pressure. In step 99, the films ofselected liquid on the objects' surfaces perform chemically squeegeeingto remove any remaining rinse liquid 27 and remaining selected liquid 37and contaminants from the objects' surfaces. In step 101 (optional), anyremaining selected liquid 37 and rinse liquid 27 are removed from thechamber. In step 103 (optional), a purge gas is passed through thechamber to remove any remaining gas and/or liquid particles from thechamber. The objects, now dried and/or cleaned, can be removed from thechamber or may be further processed in the chamber.

No matter what drain rate r1 is selected, removal of the rinse liquid 27from the chamber 11 will produce a partial vacuum within the chamberthat is not fully relieved by receipt within the chamber of the smallamount of selected liquid from the drive head 61 and vibrating nozzle63. If the chamber 11 is sufficiently air-tight, little or no gas fromthe external environment will enter the chamber in response to creationof this vacuum. However, many chambers are not sufficiently air-tight;and in such instances an appreciable amount of gas from the externalenvironment, possibly bringing with this gas one or more contaminantparticles that may settle on the exposed surfaces 14A, 14B, 14C of theselected objects 13A, 13B, 13C. This has been observed in some, but notall, of the tests of the procedure and apparatus disclosed here.

With reference to FIG. 3, a reservoir 121 of a substantially inertdisplacement gas 122, such as N₂, CO or CO₂, is optionally provided andis in fluid communication with the chamber 11. The inert gas 122 in thereservoir 121 passes through a port 123 and an associated valve andpressure control device 125 to enter the chamber 11. The valve andpressure control device 125 senses the changing pressure that is createdwithin the chamber 11 as the rinse liquid 27 is drained from the chamberusing the port and valve 41 and 43. In response to this changingpressure, the valve and pressure control device 125 allows sufficientinert gas 122 from the inert gas reservoir 121 to enter the chamber sothat the chamber pressure is maintained at a pressure p≈p(external),where p(external) is approximately equal to the local pressure externalto the chamber, or at a higher pressure. A chamber pressure p that issomewhat higher than the local external pressure p(external) ispreferred here so that some of the inert gas 122 will tend to move outof the chamber 11 into the external environment and will discouragein-flow of gases from the external environment, if the chamber is notsufficiently air-tight.

Optionally, the pressure p maintained within the chamber 11 may besomewhat less than p(external), perhaps as low as 0.8 p(external), andstill discourage entry of gas from the external environment into thechamber. After the rinse liquid 27 is fully drained from the chamber 11and the surfaces 14A, 14B and 14C of the selected objects 13A, 13B, 13Care fully dried and/or cleaned, the inert gas 122 may be removed fromthe chamber to an inert gas reservoir 127 before the next step is takenin processing the selected objects.

Alternatively, if the drain rate r1 for the rinse liquid 27 from thechamber 11 is controlled sufficiently well, the valve and pressurecontrol device 125 need not sense the internal pressure of the chamber11. In this approach, the valve and pressure control device 125 admitsinsert gas 122 at a programmed volume flow rate r3 from the inert gasreservoir 121, where the rate r3 is sufficient to maintain the internalpressure p≈p(external) or higher within the chamber 11 as the rinseliquid 27 drains from the chamber.

The temperature T of the inert gas 122 is preferably at or near thetemperature of the rinse liquid, which is usually room temperature orsomewhat colder or somewhat warmer. The purge gas reservoir 75 may alsoserve as the inert gas reservoir 121, with inclusion of the valve andpressure control device 125.

FIG. 5 illustrates improved flow filtering apparatus that is intended tokeep the selected liquid (IPA or other) relatively free fromcontaminants with sizes above a relatively small diameter. A selectedliquid (e.g., IPA or other) is held in an SL reservoir 151 and is pulledalong a first fluid-carrying line 153 and through a first check valve155 by a positive displacement pump 157 having a volume flow rate thatis preferably in the range 1-10 LPM at room temperature. The selectedliquid in the first line 153 passes through a first filter 159,preferably having a plurality of apertures with diameters in the range0.1-0.2 μm, and through a second filter 161, preferably having aplurality of apertures with diameters in the range 0.02-0.1 μm (morepreferably≈0.05 μm). The first filter 159 removes most or allcontaminant particles in the selected liquid with diameters greater thanthe first filter aperture diameter. The second filter 161 removes mostor all contaminant particles in the selected liquid with diametersgreater than the second filter aperture diameter, and more particularlyremoves any remaining contaminant particles with diameters greater thanthe first filter aperture diameter. Optionally, one of the first andsecond filters, 159 and 161, can be deleted.

The selected liquid then passes along the first line 153 to a junction163 where this first line intersects with a second fluid-carrying line165 and with a third fluid-carrying line 167. Selected liquid in thesecond line 165 passes through a second check valve 169 and is returnedto the SL reservoir 151. Selected liquid in the third line 167 passesthrough a third check valve 171 (preferably a needle valve) and througha third filter having a plurality of apertures with diameters in therange 0.02-0.1 μm (more preferably≈0.05 μm). Selected liquid in thethird line 167 is then received by a filtered SL reservoir 179 that ispressurized by inert gas from an inert gas line 181 that is fed by aninert gas source 183, which may hold N₂, CO or another suitable inertgas. The thrice-filtered selected liquid then passes through a vibratinghead and nozzle 185 for cleaning and/or drying of an object. Optionally,the SL reservoir 151 has a pressure sensor and regulator 187 that usespressure feedback to maintain approximately constant pressure in thisreservoir.

Each of the three filters, 159, 161 and 175, is preferably atrack-etched polycarbonate filter. The first and second fluid-carryinglines 153 and 165 are preferably teflon tubes with an inside diameter inthe range 0.125-0.25 mm. The third fluid-carrying line 167 is preferablya teflon tube with an inside diameter in the range 0.1-0.2 mm.

When the second check valve 169 is in the open position and the thirdcheck valve 171 is in the closed position, selected liquid passesthrough the first and second filters, 159 and 161, flows in the firstand second fluid-carrying lines, 153 and 165, and returns to the SLreservoir 151. When the second check valve 169 is in the closed positionand the third check valve 171 is in the open position, selected liquidpasses through the first, second and third filters, 159, 161 and 175,flows in the first and third fluid-carrying lines, 153 and 167, andpasses through the nozzle 185. At least one of the second and thirdcheck valves 169 and 171 is open at any time so that the filteringaction never stops and selected liquid circulates across two or three ofthe filters substantially continuously. Most of the contaminantparticles in the selected liquid are removed in the first line-secondline fluid route. Additional contaminant particles, if any are present,are removed by the first line-third line fluid route, which acts as ashunt to remove a selected fraction of the laready-filtered selectedliquid for further filtering.

FIG. 6 illustrates apparatus for suppressing or eliminating a "spike" ofcontaminant particles that appears when selected liquid (SL) deliveryapparatus is activated after a substantial period of inactivity. An SLreservoir 191 is fed by an SL fluid line 193 from an SL source 195. TheSL reservoir 191 is also pressurized by inert gas received in an inertgas line 197 from an inert gas source 199. The SL reservoir has a nozzleor other liquid outlet terminal 201 to allow SL to be delivered at acontrollable rate for cleaning and/or drying of an object. A liquid flowmask 203, having an open section 205 and an opaque section 207, islocated adjacent to the nozzle 201 and is movable in a directiontransverse to the normal direction of SL flow by a motor or other manualor automatic mask movement mechanism 209. The redirected SL liquid ispassed through one or more SL filters 208 before returning to the SLsource 195. The mask 203 acts in a manner similar to the action of afocal plane shutter in a camera and is preferably fabricated from arelatively inert material such as Gore-Tex. Any other means of liquidredirection can be used to direct an initial amount of SL through a(special) SL filter to remove most or all of a "spike" of contaminantparticles that appears when the system is first activated.

When the SL delivery apparatus, of which the SL reservoir 191 is a part,is activated after being unused for a substantial time period, theopaque portion of the mask 203 is positioned across the nozzle 201, inorder to interfere with and redirect the initial SL flow. The SL thatflows initially (immediately after the system is activated) is likely tohave a larger-than-normal number of contaminant particles therein, whichmay have accumulated in the SL delivery apparatus (SL reservoir, SLdelivery lines, etc.) during the preceding period of inactivity.Selected liquid in this initial SL flow is preferably redirected througha separate liquid filtering system (not shown in FIG. 6) to remove thelarger-than-normal number of contaminant particles from the liquid.After a selected time interval, which may be as short as a few secondsand as long as 60-120 seconds, the mask 203 is moved transversely by themovement mechanism 209 to position the open section 207 of the mask 203in the normal path of SL flow from the nozzle 201. At this point, the SLis permitted to flow from the SL reservoir 191, through the nozzle 201and through the normal channel(s) for SL delivery for cleaning and/ordrying of an object.

FIG. 7 illustrates improved aerosol production apparatus. Relativelylarge drops 211 of a selected liquid (SL) from an SL reservoir 213 aredelivered by one or more fluid-carrying lines 215 and deposited on anexposed surface of a suitable solid receptor 217. The fluid-carryingline 215 preferably allows an SL flow rate of 0.1-10 ml/min. The size ofthe SL drops may be as small as permitted by the surface tension of theSL, or larger if desired. The solid receptor 217 is preferably achemically inert material, such as silicon nitride (Si₃ N₄), siliconnitride hydride (Si_(x) N_(y) H_(z)) or any other suitable material. Thesolid receptor 217 is contiguous, on one side or at the bottom, to apiezoelectric (PZT) crystal 219. The PZT crystal 219 is electricallydriven by two or more electrodes 221A and 221B that are in turn drivenby an alternating voltage device 223. Preferably, the alternatingvoltage device 223 provides alternating voltage at one or morefrequencies in the range 20-5,000 kHz, and more preferably in the range20-750 kHz.

As the PZT crystal 219 expands and contracts in response to the imposedalternating voltage, the solid receptor 217 vibrates, and a depositeddrop of SL 211 is separated into a plurality of smaller (aerosol)particles 225 with diameters preferably in the range 1-50 μm. Thesesmaller aerosol particles 225 are thrown off of, or otherwise departfrom, the solid receptor 217 and are subsequently used for cleaningand/or drying an object. As the drive frequency f for the PZT crystal219 is increased, the average diameter of the aerosol particles producedby this apparatus should decrease.

We claim:
 1. A method for performing at least one of drying an objectand cleaning the object to remove at least one contaminant particle fromat least one exposed surface of the object, the method comprising thesteps of:placing a selected object to be dried or cleaned in an enclosedchamber; admitting a sufficient amount of a rinse liquid having aselected surface tension into the enclosed chamber so that the selectedobject is partly or fully submerged in the rinse liquid; admitting intothe enclosed chamber, at a volume flow rate r2 lying in a first selectedvolume flow range, a selected liquid that has a surface tension that issubstantially lower than the surface tension of the rinse liquid;forming aerosol particles of the selected liquid within the enclosedchamber; allowing a portion of the aerosol particles to form a film ofthe selected liquid on an exposed surface of the rinse liquid; drainingthe rinse liquid from the enclosed chamber at a volume flow rate r1lying in a second selected volume flow range, and allowing the film ofthe selected liquid to form on exposed surfaces of the selected objects;allowing the film of selected liquid to displace the rinse liquid on theexposed surfaces of the selected object, to perform at least one of (i)removing said at least one contaminant particle having a diameter of atleast 0.3 μm from said at least one exposed surface of the object and(ii) drying said at least one exposed surface of the object; anddirecting an inert gas through a nozzle into the enclosed chamber atsaid aerosol particles in one or more directions to cause the aerosolparticles to move away from a selected direction within said chamber atincreased angles relative to the selected direction, as compared tomovement of the aerosol particles when the inert gas is not present. 2.The method of claim 1, further comprising the step of removing at leastone of said aerosol particles that moves in said selected directionwithin said chamber.
 3. The method of claim 1, further comprising thestep of introducing said inert gas at a controllable, time varying gaspressure.
 4. The method of claim 1, wherein said step of forming saidaerosol particles comprises the step of passing said selected liquidthrough a vibrating nozzle that vibrates at a selected frequency f lyingin the range 10 kHz≦f≦10,000 kHz.
 5. The method of claim 1, furthercomprising the step of choosing said selected liquid to be chemicallysubstantially unreactive with said selected object.
 6. The method ofclaim 1, further comprising the step of selecting said rate r1 so thatthe depth of said rinse liquid in said enclosed chamber decreases at arate of between 3 mm/sec and 10 mm/sec.
 7. The method of claim 1,further comprising the step of selecting said rate r2 to lie in saidfirst volume flow range 1 ml/min≦r2≦5 ml/min.
 8. The method of claim 1,further comprising the step of forming substantially all of said aerosolparticles within said enclosed chamber without a change in a vapor phaseof said selected liquid.
 9. The method of claim 1, further comprisingthe step of forming said aerosol particles within said enclosed chamberwith an energy expenditure of 1.6 Watts.
 10. The method of claim 1,further comprising the step of removing substantially all said inert gasfrom said enclosed chamber, after said rinse liquid has been drainedfrom said enclosed chamber.
 11. The method of claim 1, furthercomprising the steps of:providing a measure of a pressure of anenvironment that is external to said enclosed chamber; admitting aselected displacement gas into said enclosed chamber at the time saidrinse liquid is being drained from said enclosed chamber; andcontrolling a rate of admission of the selected displacement gas intosaid enclosed chamber so that total gas pressure within said enclosedchamber is at or above the external environment pressure while saidrinse liquid is being drained from said enclosed chamber.
 12. The methodof claim 1, further comprising the step of substantially continuouslyfiltering said selected liquid to remove contaminant particles from saidselected liquid and to return most or all of said selected liquid, afterfiltering, to a reservoir for said selected liquid.
 13. The method ofclaim 1, further comprising the steps of:redirecting all flow of saidselected liquid for a selected time interval, during which time saidselected liquid is not being admitted into said enclosed chamber;filtering said selected liquid that is redirected; and allowing saidselected liquid to flow without interference after the selected timeinterval and to be used to form said aerosol particles.
 14. The methodof claim 1, further comprising the step of performing, at a temperaturethat is approximately room temperature, at least one of said steps offorming said aerosol particles, allowing said portion of said aerosolparticles to form said film of said selected liquid, draining said rinseliquid from said enclosed chamber, allowing said film of said selectedliquid to form on said exposed surfaces of said selected object, andallowing said film of said selected liquid to displace said rinse liquidon said exposed surfaces of said selected object.
 15. The method ofclaim 3, further comprising the step of varying said time varying gaspressure to produce a gas pressure that varies between a selectedminimum gas pressure and a selected maximum gas pressure.
 16. The methodof claim 4, further comprising said step of selecting said frequency fto lie in a range 20 kHz≦f≦100 kHz.
 17. The method of claim 4, furthercomprising the step of selecting said frequency f so that at least oneof said aerosol particles has an estimated diameter d(sel) that lies inthe range 10 μm≦d(sel)≦50 μm.
 18. The method of claim 5, furthercomprising the step of choosing said selected liquid from a group ofsubstantially unreactive liquids consisting of isopropyl alcohol, ethylalcohol, methyl alcohol, tetrahydrofuran, acetone, perfluorohexane,hexane and ether.
 19. The method of claim 10, further comprising thestep of passing a heated selected purge gas through said enclosedchamber after said rinse liquid has been drained from said enclosedchamber.
 20. The method of claim 12, further comprising the stepsof:receiving a selected fraction of said filtered selected liquid andapplying further filtering to said liquid received; and using saidselected liquid that has been further filtered to form said aerosolparticles.
 21. A method for performing at least one of drying an objectand cleaning the object to remove at least one contaminant particle fromat least one exposed surface of the object, the method comprising thesteps of:placing a selected object to be dried in an enclosed chamber;admitting a sufficient amount of a rinse liquid having a selectedsurface tension into the enclosed chamber so that the selected object ispartly or fully submerged in the rinse liquid; admitting into theenclosed chamber, at a volume flow rate r2 lying in a first selectedvolume flow range, a selected liquid that has a surface tension that issubstantially lower than the surface tension of the rinse liquid;forming aerosol particles of the selected liquid within the enclosedchamber; allowing a portion of the aerosol particles to form a film ofthe selected liquid on an exposed surface of the rinse liquid; drainingthe rinse liquid from the enclosed chamber at a volume flow rate r1lying in a second selected volume flow range, and allowing the film ofthe selected liquid to form on exposed surfaces of the selected object;allowing the film of selected liquid to displace the rinse liquid on theexposed surfaces of the selected object, to perform at least one of (i)removing said at least one contaminant particle having a diameter of atleast 0.3 μm from said at least one exposed surface of the object and(ii) drying said at least one exposed surface of the object; admitting aselected displacement gas into said enclosed chamber at the time therinse liquid is being drained from the enclosed chamber; providing ameasure of a pressure of an environment that is external to saidenclosed chamber; and controlling a rate of admission of the selecteddisplacement gas into the enclosed chamber so that total gas pressurewithin the enclosed chamber is at or above the external environmentpressure while the rinse liquid is being drained from the enclosedchamber.
 22. The method of claim 21, further comprising the step ofsubstantially continuously filtering said selected liquid to remove saidcontaminant particles from said selected liquid and to return most orall of said selected liquid, after filtering, to a reservoir for saidselected liquid.
 23. The method of claim 21, further comprising the stepof performing, at a temperature that is approximately room temperature,at least one of said steps of forming said aerosol particles, allowingsaid portion of said aerosol particles to form said film of saidselected liquid, draining said rinse liquid from said enclosed chamber,allowing said film of said selected liquid to form on said exposedsurfaces of said selected object, and allowing said film of saidselected liquid to displace said rinse liquid on said exposed surfacesof said selected object.